CN110660977B - Lithium-sulfur electrochemical energy storage system and preparation method thereof - Google Patents

Lithium-sulfur electrochemical energy storage system and preparation method thereof Download PDF

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CN110660977B
CN110660977B CN201910784790.XA CN201910784790A CN110660977B CN 110660977 B CN110660977 B CN 110660977B CN 201910784790 A CN201910784790 A CN 201910784790A CN 110660977 B CN110660977 B CN 110660977B
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sulfur
graphene
lithium
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active material
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CN110660977A (en
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王晓敏
赵振新
杨哲伟
李慧君
邱小明
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Taiyuan University of Technology
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    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
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    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
    • H01M4/5805—Phosphides
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Abstract

The invention discloses a lithium-sulfur electrochemical energy storage system which comprises a sulfur-containing anode, a lithium-containing cathode, a diaphragm and electrolyte, wherein the sulfur-containing anode comprises an anode active material, the lithium-containing cathode comprises a cathode active material, and the anode active material is sulfur/graphene/MPxThe negative active material is graphene/MPxNanocomposite materials, wherein, MPxIs transition metal phosphide, and the mass ratio of the positive electrode active material to the negative electrode active material is 1: (1.4-1.6). The invention also provides a preparation method of the lithium-sulfur electrochemical energy storage system. The lithium-sulfur electrochemical energy storage system has the advantages of long service life, high safety performance, excellent electrochemical performance and the like.

Description

Lithium-sulfur electrochemical energy storage system and preparation method thereof
Technical Field
The invention belongs to the field of energy storage devices, and particularly relates to an energy storage system and a preparation method thereof.
Background
As an intermediary for energy storage and conversion, lithium ion batteries have been widely used in portable electric appliances, mobile devices, electric vehicles, and the like for the past several decades. Along with the development of society, the demand of human beings on energy rises sharply, and the lithium ion battery can not meet the requirements of the current society on energy storage devices in the aspect of energy density. Therefore, a reasonable energy structure change is the key to solve the problem. Among them, the lithium-sulfur battery is one of the international research hotspots in the energy storage field due to its high energy density and low cost, and its mass energy density is 3-5 times of the existing commercial lithium-ion battery system.
For a mature energy storage system, the high energy density, high power density, excellent safety, long cycle stability, long shelf life and the like are evaluatedImportant parameters of the performance. However, besides high energy density, lithium-sulfur batteries have a certain gap in other aspects compared with current lithium-ion batteries. The difference in performance depends mainly on the following problems: (1) low sulfur conductivity (5.0X 10)-30S/cm); (2) the charge-discharge intermediate product can be dissolved in the electrolyte and shuttled back and forth between the positive electrode and the negative electrode to cause the damage of the inherent structure of the electrodes; (3) elemental sulfur expands up to 79% in volume during charging and discharging. (4) Lithium dendrites formed during charging and discharging of the metallic lithium negative electrode may cause a serious safety problem.
Disclosure of Invention
The technical problem to be solved by the invention is to overcome the defects and shortcomings in the background technology, and provide a lithium-sulfur electrochemical energy storage system with long service life, high safety performance and excellent electrochemical performance and a preparation method thereof. In order to solve the technical problems, the technical scheme provided by the invention is as follows:
a lithium-sulfur electrochemical energy storage system comprises a sulfur-containing positive electrode, a lithium-containing negative electrode, a diaphragm and electrolyte, wherein the sulfur-containing positive electrode comprises a positive active material, the lithium-containing negative electrode comprises a negative active material, and the positive active material is sulfur/graphene/MPxThe negative active material is graphene/MPxNanocomposite materials, wherein, MPxIs transition metal phosphide, and the mass ratio of the positive electrode active material to the negative electrode active material is 1: (1.4-1.6).
According to the lithium-sulfur electrochemical energy storage system, the positive electrode and the negative electrode adopt the same preparation method, so that the production cost can be reduced. The anode and the cathode are optimized through material selection, the content of active substances in the anode and cathode materials is optimized and selected (controlled to be 1 (1.4-1.6)), the anode and the cathode play different action mechanisms, the materials are mutually cooperated, the high energy density of the whole electrode is more easily played after the capacities of the anode and the cathode are matched, and the lithium-sulfur electrochemical energy storage system with excellent performance can be obtained.
The diaphragm is a commercial PP diaphragm, and the electrolyte is LiTFSI dissolved in a mixed electrolyte of DME and DOL in a certain ratio (DME: DOL is 1: 1 Vol%). The capacity of the lithium-sulfur electrochemical energy storage system is derived from the positive electrodeMulti-electron reaction of sulfur and negative electrode MPxA transformation reaction or an alloying reaction. In the discharging process, lithium ions are removed from the negative electrode and enter the electrolyte to react with the positive electrode sulfur; during charging, lithium ions are extracted from the anode and react with the cathode MPxA conversion reaction occurs, and the process realizes the storage and conversion of energy. The charging and discharging voltage interval of the lithium-sulfur electrochemical energy storage system device is 0.6-2.3V.
In the lithium-sulfur electrochemical energy storage system, preferably, M includes any one of Fe, Co, Ni, Cu, and Mn, and MP isxIn particular FeP, CoP and Ni2P、Cu3P and MnP.
In the lithium-sulfur electrochemical energy storage system, preferably, the sulfur/graphene/MPxIn the nano composite material, the transition metal phosphide is of a nano spherical structure, the diameter of the transition metal phosphide is 80-120nm, the transition metal phosphide is uniformly loaded on a graphene substrate, and the sulfur is wrapped in a three-dimensional cavity of the graphene.
In the lithium-sulfur electrochemical energy storage system, preferably, the graphene/MPxIn the nano composite material, the transition metal phosphide is in a nano spherical structure, the diameter of the transition metal phosphide is 80-120nm, and the transition metal phosphide is uniformly loaded on a graphene substrate.
In the lithium-sulfur electrochemical energy storage system, preferably, the sulfur/graphene/MPxIn the nano composite material, the proportional relation (the ratio of the molar weight to the mass) of the transition metal M and the graphene (corresponding to the mass of the reduced graphene oxide slices) is (0.3-1.5 mol): (40-80 mg); the graphene/MPxIn the nano composite material, the proportion relation of the transition metal M and the graphene is (3-8 mol): (40-80 mg). The research shows that the principle of the transition metal phosphide playing a role in the positive electrode and the negative electrode is different, the positive electrode utilizes the catalytic adsorption characteristic of the transition metal phosphide, the dosage is too small to achieve an ideal catalytic effect, and the dosage is too large to influence the conductivity of the whole electrode material, so that the rate capability of the material is low. The negative electrode utilizes the high capacity characteristic of the transition metal phosphide, the dosage is too small to reach the ideal capacity, and the dosage is too large, so that the volume expansion of the transition metal phosphide causes the damage of an electrode material during the charge and discharge. And, due to the space between the anode and cathode materialsThe method has obvious mutual influence relationship, the effect exerted by the two functions can not be influenced by each other, and when the dosage of the transition metal phosphide in the positive electrode and the negative electrode is determined, the influence relationship between the positive electrode and the negative electrode and the dosage ratio of the active materials in the positive electrode and the negative electrode need to be considered. By the use amount ratio of the transition metal M to the graphene, the content of the transition metal phosphide in the positive electrode and the negative electrode can be controlled, and the positive electrode and the negative electrode active materials with excellent performance and matched performance can be obtained.
As a general technical concept, the present invention also provides a method for preparing a lithium sulfur electrochemical energy storage system, comprising the steps of:
(1) method for respectively preparing positive active material sulfur/graphene/MP by two-step hydrothermal methodxNanocomposite and negative active material graphene/MPxNanocomposite of sulfur/graphene/MPxMixing the nano composite material with a conductive agent and an additive, pulping, coating to obtain a sulfur-containing anode, and then mixing graphene/MPxMixing the nano composite material with a conductive agent and an additive to prepare slurry, coating and pre-lithiating to obtain a lithium-containing negative electrode;
(2) and assembling a sulfur-containing positive electrode, a lithium-containing negative electrode, a diaphragm and electrolyte to obtain the lithium-sulfur electrochemical energy storage system.
In the preparation method, the preferable two-step hydrothermal method is used for preparing the positive active material sulfur/graphene/MPxThe nanocomposite comprises the following steps:
(1) preparing reduced graphene oxide hydrogel by using a hydrothermal reaction, and slicing the reduced graphene oxide hydrogel to obtain reduced graphene oxide slices;
(2) adding ammonia water into the salt solution containing M, then adding reduced graphene oxide slices, standing, then heating for reaction, washing after the reaction is finished, and freeze-drying to obtain an intermediate 1; controlling the proportion relation of the transition metal M in the M salt and the reduced graphene oxide to be (0.3-1.5 mol): (40-80 mg);
(3) mixing the intermediate 1 with sodium hypophosphite, heating to react in an inert atmosphere, washing and drying after the reaction is finished to obtain the graphene/MPxA nanocomposite;
(4) mixing graphene/MPxThe mass ratio of the nano composite material to the sublimed sulfur is 1: (2-5) after being uniformly mixed, the mixture is heated in vacuum and cooled to obtain the sulfur/graphene/MPxA nanocomposite material. The heating temperature is controlled to be 140-160 ℃ during vacuum heating, and the heat preservation time is 12-24 h.
In the preparation method, the preferable two-step hydrothermal method is used for preparing the negative active material graphene/MPxThe nanocomposite comprises the following steps:
(1) preparing reduced graphene oxide hydrogel by using a hydrothermal reaction, and slicing the reduced graphene oxide hydrogel to obtain reduced graphene oxide slices;
(2) adding ammonia water into the salt solution containing M, then adding reduced graphene oxide slices, standing, then heating for reaction, washing after the reaction is finished, and freeze-drying to obtain an intermediate 2; controlling the proportion relation of the transition metal M in the M salt and the reduced graphene oxide to be (3-8 mol): (40-80 mg);
(3) mixing the intermediate 2 with sodium hypophosphite, heating to react in an inert atmosphere, washing and drying after the reaction is finished to obtain the graphene/MPxA nanocomposite material.
In the preparation method, preferably, when the reduced graphene oxide hydrogel is prepared by hydrothermal reaction, the concentration of the graphene oxide in the reaction system is controlled to be 1-4mg/mL, the reaction temperature is 140-200 ℃, and the reaction time is 10-24 h.
In the preparation method, preferably, after standing, the reaction is heated, the reaction temperature is controlled to be 120-180 ℃, and the reaction time is controlled to be 3-12 h; when the reaction is heated in the inert atmosphere, the reaction temperature is controlled to be 300-400 ℃, the heat preservation time is 1-4h, and the heating rate is 5 ℃/min.
In the above production method, preferably, the M salt is any one of ferric chloride, ferric nitrate, cobalt chloride, cobalt nitrate, nickel chloride, nickel nitrate, cupric chloride, cupric nitrate, manganese chloride, and manganese nitrate.
In the above preparation method, preferably, the mass ratio of the intermediate 1 or the intermediate 2 to the sodium hypophosphite is controlled to be 1: (10-20).
In the invention, a two-step hydrothermal method is adopted to prepare the anode active materialSulfur/graphene/MPxNanocomposite and negative active material graphene/MPxThe nano composite material is prepared by preparing reduced graphene oxide gel and then taking the reduced graphene oxide gel as a matrix growth precursor. The two-step hydrothermal method can better reduce graphene oxide, improve the overall conductivity, is beneficial to fully exerting the rate capability of the lithium-sulfur battery, and has more excellent electrochemical performance.
In the invention, the material is prepared from sulfur/graphene/MPxThe nano composite material, the conductive agent and the additive are mixed according to the mass ratio of 7: 2: 1, adding NMP dropwise and stirring to obtain anode membrane slurry, coating the anode membrane slurry on aluminum foil, and drying for 24 hours in vacuum at 60 ℃ to obtain the anode. From graphene/MPxThe nano composite material, the conductive agent and the additive are mixed according to the mass ratio of 8: 1: 1, adding NMP dropwise and stirring to obtain negative electrode film slurry, coating the negative electrode film slurry on copper foil, drying the copper foil in vacuum at 120 ℃ for 3 hours, drying the copper foil in vacuum at 60 ℃ for 12 hours, and finally carrying out pre-lithiation to obtain the negative electrode. Wherein, the prelithiation adopts electrochemical prelithiation, lithium powder prelithiation and the like.
According to the invention, the reduced graphene oxide with high conductivity is prepared by a two-step hydrothermal method, so that the technical problem of low sulfur conductivity can be solved. The catalytic adsorption characteristic of the transition metal phosphide is utilized in the positive active material, the transition metal phosphide can adsorb polysulfide and relieve the polysulfide from being dissolved into electrolyte, and the transition metal phosphide can catalyze the polysulfide conversion so that the polysulfide can be quickly converted into Li2S, the shuttle effect can be relieved by the two characteristics; the volume expansion of sulfur can be relieved by utilizing the physical confinement and chemical adsorption effect of the graphene on the sulfur.
Compared with the prior art, the invention has the advantages that:
1. the sulfur-containing positive electrode of the lithium-sulfur electrochemical energy storage system comprises a positive active material, the lithium-containing negative electrode comprises a negative active material, the positive electrode utilizes the physical confinement of three-dimensional graphene to sulfur and the catalytic adsorption effect of transition metal phosphide to polysulfide, and the negative electrode combines the high conductivity of the three-dimensional graphene and the high lithium storage capacity of the transition metal phosphide, so that the battery system has the outstanding advantages of safety and stable circulation, and the electrochemical performance can be greatly improved.
2. According to the invention, the original metal lithium foil is replaced by the high-capacity graphene/MPx cathode, so that the material cost can be greatly reduced, and the safety problems of lithium dendrite and the like caused by the lithium foil can be solved.
3. The preparation method of the invention is simple, the raw materials are easy to obtain, and the reproducibility is good.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to these drawings without creative efforts.
Fig. 1 is an SEM image of intermediate 1 in example 1.
Fig. 2 is an SEM image of intermediate 2 in example 1.
FIG. 3 shows the sulfur/graphene/Ni containing material prepared in example 22Charge and discharge curves for the positive electrode and the lithium negative electrode of P.
FIG. 4 is the graphene/Ni prepared in example 22P and lithium charge-discharge curves for the electrode half-cell.
Fig. 5 is a charge-discharge curve diagram of a lithium-sulfur battery assembled by the sulfur/graphene/FeP-containing positive electrode and the graphene/FeP-containing negative electrode prepared in example 3.
Fig. 6 is a graph showing cycle performance of the lithium sulfur battery prepared in example 3.
Detailed Description
In order to facilitate an understanding of the invention, the invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of the invention is not limited to the specific embodiments below.
Unless otherwise defined, all terms of art used hereinafter have the same meaning as commonly understood by one of ordinary skill in the art. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of the present invention.
Unless otherwise specifically stated, various raw materials, reagents, instruments, equipment and the like used in the present invention are commercially available or can be prepared by existing methods.
Example 1:
a lithium-sulfur electrochemical energy storage system comprises a sulfur-containing positive electrode, a lithium-containing negative electrode, a diaphragm and electrolyte. The sulfur-containing positive electrode includes a positive active material, and the lithium-containing negative electrode includes a negative active material. The positive electrode active material is a sulfur/graphene/CoP nano composite material, and the negative electrode active material is a graphene/CoP nano composite material. The mass of the positive electrode active material was 0.9mg/cm2The mass ratio of the negative electrode active material was 1.28mg/cm2。
The preparation method of the lithium-sulfur electrochemical energy storage system comprises the following steps: preparing a sulfur/graphene/CoP nanocomposite material and a graphene/CoP nanocomposite material; and adjusting the dosage ratio of the anode and the cathode, and assembling the anode, the cathode, the diaphragm and the electrolyte to obtain the lithium-sulfur electrochemical energy storage system.
The preparation method of the positive electrode comprises the following steps:
(1) preparing graphene by adopting improved Hummer's, carrying out ultrasonic treatment on the prepared graphite oxide to prepare a 4mg/mL solution, carrying out hydrothermal reaction for 12 hours at 180 ℃ in a reaction kettle to prepare reduced graphene oxide hydrogel, and slicing the reduced graphene oxide hydrogel to obtain reduced graphene oxide slices;
(2) 1mmol of CoCl2·6H2Dissolving O in 50mL of ethanol, then dropwise adding 1mL of ammonia water, uniformly stirring, then adding 40mg of reduced graphene oxide slices, standing for 6 hours, then heating for reaction, controlling the reaction temperature to be 180 ℃, reacting for 6 hours, washing after the reaction is finished, and freeze-drying to obtain an intermediate 1;
(3) and then mixing the intermediate 1 with sodium hypophosphite according to a mass ratio of 1: 10, uniformly mixing, heating at 400 ℃ for reaction for 2h in an inert atmosphere, controlling the heating rate to be 5 ℃/min, and washing and drying after the reaction is finished to obtain the graphene/CoP nano composite material;
(4) mixing a graphene/CoP nano composite material with sublimed sulfur according to a mass ratio of 1: 3, uniformly mixing, heating in vacuum at 155 ℃ for 12h, and cooling to obtain the sulfur/graphene/MPx nano composite material;
(5) mixing a sulfur/graphene/CoP nano composite material, a conductive agent and activated carbon in a weight ratio of 7: 2: 1, adding NMP dropwise, stirring into slurry, coating the slurry on an aluminum foil, and drying in vacuum at 60 ℃ for 24 hours to obtain the anode.
The preparation method of the negative electrode comprises the following steps:
(1) preparing graphene by adopting improved Hummer's, carrying out ultrasonic treatment on the prepared graphite oxide to prepare a 4mg/mL solution, carrying out hydrothermal reaction for 12 hours at 180 ℃ in a reaction kettle to prepare reduced graphene oxide hydrogel, and slicing the reduced graphene oxide hydrogel to obtain reduced graphene oxide slices;
(2) 3mmol of Co (NO)3)2·6H2Dissolving O in 50mL of ethanol, then dropwise adding 1mL of ammonia water, uniformly stirring, then adding 40mg of reduced graphene oxide slices, standing for 6 hours, then heating for reaction, controlling the reaction temperature to be 180 ℃, reacting for 6 hours, washing after the reaction is finished, and freeze-drying to obtain an intermediate 2;
(3) and then mixing the intermediate 2 with sodium hypophosphite according to a mass ratio of 1: 20, uniformly mixing, heating at 400 ℃ for reaction for 2h in an inert atmosphere, controlling the heating rate to be 5 ℃/min, and washing and drying after the reaction is finished to obtain the graphene/CoP nano composite material;
(4) mixing the graphene/CoP nano composite material, the conductive agent and the activated carbon in a ratio of 8: 1: 1, dripping NMP, stirring into slurry, coating the slurry on a copper foil, drying at 120 ℃ for 3 hours in vacuum, and then drying at 60 ℃ for 12 hours in vacuum to obtain a negative electrode film;
(5) and (3) uniformly coating proper lithium powder on the negative electrode film, and then compacting to ensure that the lithium powder is in close contact with the negative electrode film to obtain the negative electrode.
The positive and negative electrode capacity matching is realized by adjusting the mass ratio of the positive and negative electrode active substances, and the full-cell (lithium-sulfur cell) is obtained by assembling the positive electrode, the negative electrode, the diaphragm and the electrolyte. The diaphragm is a commercial PP diaphragm, and the electrolyte is LiTFSI dissolved in a mixed electrolyte of DME and DOL in a certain ratio (DME: DOL is 1: 1 Vol%).
Fig. 1 is an SEM image of the graphene/CoP composite material (i.e., intermediate 1) when the positive electrode of this example is prepared, and it can be seen from the SEM image that the prepared CoP is about 100nm and the particles are uniformly distributed on the graphene sheet. Fig. 2 is an SEM image of the graphene/CoP composite material (i.e., intermediate 2) during the preparation of the negative electrode, and it can be seen from the SEM image that the amount of CoP during the preparation of the negative electrode is much higher than that of the positive electrode because the principle of CoP utilized by the positive and negative electrodes is different.
Example 2:
a lithium-sulfur electrochemical energy storage system comprises a sulfur-containing positive electrode, a lithium-containing negative electrode, a diaphragm and electrolyte. The sulfur-containing positive electrode includes a positive active material, and the lithium-containing negative electrode includes a negative active material. The positive active material is sulfur/graphene/Ni2P nano composite material, negative active material is graphene/Ni2P nanocomposite. The mass of the positive electrode active material was 0.88mg/cm2The mass ratio of the negative electrode active material was 1.33mg/cm2。
The preparation method of the lithium-sulfur electrochemical energy storage system comprises the following steps: preparation of sulfur/graphene/Ni2P nanocomposite and graphene/Ni2A P nanocomposite; and adjusting the dosage ratio of the anode and the cathode, and assembling the anode, the cathode, the diaphragm and the electrolyte to obtain the lithium-sulfur electrochemical energy storage system.
The preparation method of the positive electrode comprises the following steps:
(1) preparing graphene by adopting improved Hummer's, carrying out ultrasonic treatment on prepared graphite oxide to prepare a solution of 2mg/mL, carrying out hydrothermal reaction for 12 hours at 180 ℃ in a reaction kettle to prepare reduced graphene oxide hydrogel, and slicing the reduced graphene oxide hydrogel to obtain reduced graphene oxide slices;
(2) 1.5mmol of Ni (NO)3)2·6H2Dissolving O in 50mL of ethanol, then dropwise adding 1.5mL of ammonia water, uniformly stirring, then adding 40mg of reduced graphene oxide slices, standing for 6 hours, then heating for reaction, controlling the reaction temperature to be 180 ℃, reacting for 12 hours, washing after the reaction is finished, and freeze-drying to obtain an intermediate 1;
(3) then the intermediate 1 is mixed with sodium hypophosphiteThe quantity ratio is 1: 20, uniformly mixing, heating and reacting for 2 hours at 300 ℃ in an inert atmosphere, controlling the temperature rise rate to be 5 ℃/min, washing and drying after the reaction is finished to obtain the graphene/Ni2A P nanocomposite;
(4) mixing graphene/Ni2The mass ratio of the P nano composite material to the sublimed sulfur is 1: 3, uniformly mixing, heating for 12 hours in vacuum at 155 ℃, and cooling to obtain the sulfur/graphene/Ni2A P nanocomposite;
(5) mixing sulfur/graphene/Ni2The P nanocomposite, the conductive agent and the activated carbon were mixed in a ratio of 7: 2: 1, adding NMP dropwise, stirring into slurry, coating the slurry on an aluminum foil, and drying in vacuum at 60 ℃ for 24 hours to obtain the anode.
The preparation method of the negative electrode comprises the following steps:
(1) preparing graphene by adopting improved Hummer's, carrying out ultrasonic treatment on prepared graphite oxide to prepare a solution of 2mg/mL, carrying out hydrothermal reaction for 12 hours at 180 ℃ in a reaction kettle to prepare reduced graphene oxide hydrogel, and slicing the reduced graphene oxide hydrogel to obtain reduced graphene oxide slices;
(2) adding 5mmol of Ni (NO)3)2·6H2Dissolving O in 50mL of ethanol, then dropwise adding 1mL of ammonia water, uniformly stirring, then adding 40mg of reduced graphene oxide slices, standing for 6 hours, then heating for reaction, controlling the reaction temperature to be 180 ℃, reacting for 12 hours, washing after the reaction is finished, and freeze-drying to obtain an intermediate 2;
(3) and then mixing the intermediate 2 with sodium hypophosphite according to a mass ratio of 1: 20, uniformly mixing, heating and reacting for 2 hours at 300 ℃ in an inert atmosphere, controlling the temperature rise rate to be 5 ℃/min, washing and drying after the reaction is finished to obtain the graphene/Ni2A P nanocomposite;
(4) mixing graphene/Ni2The P nanocomposite, the conductive agent and the activated carbon are mixed in a ratio of 8: 1: 1, dripping NMP, stirring into slurry, coating the slurry on a copper foil, drying at 120 ℃ for 3 hours in vacuum, and then drying at 60 ℃ for 12 hours in vacuum to obtain a negative electrode film;
(5) and (3) uniformly coating proper lithium powder on the negative electrode film, and then compacting to ensure that the lithium powder is in close contact with the negative electrode film to obtain the negative electrode.
The positive and negative electrode capacity matching is realized by adjusting the mass ratio of the positive and negative electrode active substances, and the full-cell (lithium-sulfur cell) is obtained by assembling the positive electrode, the negative electrode, the diaphragm and the electrolyte. The diaphragm is a commercial PP diaphragm, and the electrolyte is LiTFSI dissolved in a mixed electrolyte of DME and DOL in a certain ratio (DME: DOL is 1: 1 Vol%).
FIG. 3 shows the sulfur/graphene/Ni-containing material prepared in this example2The charge-discharge curves of the positive electrode and the negative electrode of P show two plateaus at about 2.4V and about 2.1V, respectively corresponding to the conversion of sulfur to polysulfide (Li)2S6、Li2S4) And polysulfide to Li2And (S). FIG. 4 shows the graphene/Ni prepared in this example2The charge-discharge curves of P and lithium on the electrode half-cell, as can be seen from the figure, have a plateau around 0.6V contributing most of the capacity, which corresponds to Ni2Transformation reaction of P with Li. Fig. 3 and 4 illustrate that the positive and negative electrodes each have good electrochemical performance when used alone.
Example 3:
a lithium-sulfur electrochemical energy storage system comprises a sulfur-containing positive electrode, a lithium-containing negative electrode, a diaphragm and electrolyte. The sulfur-containing positive electrode includes a positive active material, and the lithium-containing negative electrode includes a negative active material. The positive electrode active material is a sulfur/graphene/FeP nano composite material, and the negative electrode active material is a graphene/FeP nano composite material. The mass of the positive electrode active material was 0.88mg/cm2The mass ratio of the negative electrode active material was 1.33mg/cm2。
The preparation method of the lithium-sulfur electrochemical energy storage system comprises the following steps: preparing a sulfur/graphene/FeP nano composite material and a graphene/FeP nano composite material; and adjusting the dosage ratio of the anode and the cathode, and assembling the anode, the cathode, the diaphragm and the electrolyte to obtain the lithium-sulfur electrochemical energy storage system.
The preparation method of the positive electrode comprises the following steps:
(1) preparing graphene by adopting improved Hummer's, carrying out ultrasonic treatment on prepared graphite oxide to prepare a solution of 2mg/mL, carrying out hydrothermal reaction in a reaction kettle at 180 ℃ for 24 hours to prepare reduced graphene oxide hydrogel, and slicing the reduced graphene oxide hydrogel to obtain reduced graphene oxide slices;
(2) 1.5mmol of Fe (NO)3)3·6H2Dissolving O in 50mL of ethanol, then dropwise adding 1.5mL of ammonia water, uniformly stirring, then adding 40mg of reduced graphene oxide slices, standing for 6 hours, then heating for reaction, controlling the reaction temperature to be 180 ℃, reacting for 12 hours, washing after the reaction is finished, and freeze-drying to obtain an intermediate 1;
(3) and then mixing the intermediate 1 with sodium hypophosphite according to a mass ratio of 1: 10, uniformly mixing, heating and reacting at 350 ℃ for 2h in an inert atmosphere, controlling the heating rate to be 5 ℃/min, and washing and drying after the reaction to obtain the graphene/FeP nano composite material;
(4) mixing the graphene/FeP nano composite material with sublimed sulfur according to a mass ratio of 1: 3, uniformly mixing, heating in vacuum at 155 ℃ for 12h, and cooling to obtain the sulfur/graphene/FeP nano composite material;
(5) mixing a sulfur/graphene/FeP nano composite material, a conductive agent and activated carbon in a ratio of 7: 2: 1, adding NMP dropwise, stirring into slurry, coating the slurry on an aluminum foil, and drying in vacuum at 60 ℃ for 24 hours to obtain the anode.
The preparation method of the negative electrode comprises the following steps:
(1) preparing graphene by adopting improved Hummer's, carrying out ultrasonic treatment on prepared graphite oxide to prepare a solution of 2mg/mL, carrying out hydrothermal reaction for 12 hours at 180 ℃ in a reaction kettle to prepare graphene oxide hydrogel, and slicing the graphene oxide hydrogel to obtain graphene oxide slices;
(2) adding 5mmol of Fe (NO)3)3·6H2Dissolving O in 50mL of ethanol, then dropwise adding 1mL of ammonia water, uniformly stirring, then adding 40mg of graphene oxide slices, standing for 6 hours, then heating for reaction, controlling the reaction temperature to be 180 ℃, reacting for 12 hours, washing after the reaction is finished, and freeze-drying to obtain an intermediate 2;
(3) and then mixing the intermediate 2 with sodium hypophosphite according to a mass ratio of 1: 10, uniformly mixing, heating and reacting at 350 ℃ for 2h in an inert atmosphere, controlling the heating rate to be 5 ℃/min, and washing and drying after the reaction to obtain the graphene/FeP nano composite material;
(4) mixing the graphene/FeP nano composite material, the conductive agent and the activated carbon in a ratio of 8: 1: 1, dripping NMP, stirring into slurry, coating the slurry on a copper foil, drying at 120 ℃ for 3 hours in vacuum, and then drying at 60 ℃ for 12 hours in vacuum to obtain a negative electrode film;
(5) after the negative electrode film and the lithium sheet are assembled into a half battery, the voltage is discharged to 0.01V by adopting an electrochemical pre-lithiation method to cut off, then the battery is disassembled, and the pre-lithiated negative electrode film and the positive electrode are assembled into a full battery.
The positive and negative electrode capacity matching is realized by adjusting the mass ratio of the positive and negative electrode active substances, and the full-cell (lithium-sulfur cell) is obtained by assembling the positive electrode, the negative electrode, the diaphragm and the electrolyte. The diaphragm is a commercial PP diaphragm, and the electrolyte is LiTFSI dissolved in a mixed electrolyte of DME and DOL in a certain ratio (DME: DOL is 1: 1 Vol%).
Fig. 5 is a charge-discharge curve diagram of the novel lithium-sulfur battery with the sulfur/graphene/FeP-containing positive electrode and the graphene/FeP-containing negative electrode prepared in this example, and it can be seen from the graph that the discharge capacity at the first cycle is very high due to the activation process at the first cycle after pre-lithiation. Fig. 6 is a graph showing the cycle performance of the lithium-sulfur battery prepared in this example, and it can be seen that the cycle performance of the lithium-sulfur battery in this example is excellent.

Claims (5)

1. The lithium-sulfur electrochemical energy storage system comprises a sulfur-containing positive electrode, a lithium-containing negative electrode, a diaphragm and electrolyte, wherein the sulfur-containing positive electrode comprises a positive active material, and the lithium-containing negative electrode comprises a negative active material, and is characterized in that the positive active material is sulfur/graphene/MPxThe negative active material is graphene/MPxNanocomposite materials, wherein, MPxIs transition metal phosphide, and the mass ratio of the positive electrode active material to the negative electrode active material is 1: (1.4-1.6);
the sulfur/graphene/MPxIn the nano composite material, the transition metal phosphide is in a nano spherical structure, the diameter of the transition metal phosphide is 80-120nm, the transition metal phosphide is uniformly loaded on a graphene substrate, and the sulfur is coated on the stoneA three-dimensional cavity of graphene;
the graphene/MPxIn the nano composite material, the transition metal phosphide is in a nano spherical structure, the diameter of the transition metal phosphide is 80-120nm, and the transition metal phosphide is uniformly loaded on a graphene substrate;
the sulfur/graphene/MPxIn the nano composite material, the proportion relation of the transition metal M and the reduced graphene oxide is (0.3-1.5 mol): (40-80 mg); the graphene/MPxIn the nano composite material, the proportion relation of the transition metal M and the reduced graphene oxide is (3-8 mol): (40-80 mg).
2. The lithium sulfur electrochemical energy storage system of claim 1, wherein M comprises any one of Fe, Co, Ni, Cu, Mn, and MPxIn particular FeP, CoP and Ni2P、Cu3P and MnP.
3. A preparation method of a lithium-sulfur electrochemical energy storage system is characterized by comprising the following steps:
(1) method for respectively preparing positive active material sulfur/graphene/MP by two-step hydrothermal methodxNanocomposite and negative active material graphene/MPxNanocomposite of sulfur/graphene/MPxMixing the nano composite material with a conductive agent and an additive, pulping, coating to obtain a sulfur-containing anode, and then mixing graphene/MPxMixing the nano composite material with a conductive agent and an additive to prepare slurry, coating and pre-lithiating to obtain a lithium-containing negative electrode;
(2) assembling a sulfur-containing positive electrode, a lithium-containing negative electrode, a diaphragm and electrolyte to obtain the lithium-sulfur electrochemical energy storage system;
preparation of positive active material sulfur/graphene/MP by two-step hydrothermal methodxThe nanocomposite comprises the following steps:
(1) preparing reduced graphene oxide hydrogel by using a hydrothermal reaction, and slicing the reduced graphene oxide hydrogel to obtain reduced graphene oxide slices;
(2) adding ammonia water into the salt solution containing M, then adding reduced graphene oxide slices, standing, then heating for reaction, washing after the reaction is finished, and freeze-drying to obtain an intermediate 1; controlling the proportion relation of the transition metal M in the M salt and the reduced graphene oxide to be (0.3-1.5 mol): (40-80 mg);
(3) mixing the intermediate 1 with sodium hypophosphite, heating to react in an inert atmosphere, washing and drying after the reaction is finished to obtain the graphene/MPxA nanocomposite;
(4) mixing graphene/MPxUniformly mixing the nano composite material with sublimed sulfur, heating in vacuum, and cooling to obtain sulfur/graphene/MPxA nanocomposite;
two-step hydrothermal method for preparing negative active material graphene/MPxThe nanocomposite comprises the following steps:
(1) preparing reduced graphene oxide hydrogel by using a hydrothermal reaction, and slicing the reduced graphene oxide hydrogel to obtain reduced graphene oxide slices;
(2) adding ammonia water into the salt solution containing M, then adding reduced graphene oxide slices, standing, then heating for reaction, washing after the reaction is finished, and freeze-drying to obtain an intermediate 2; controlling the proportion relation of the transition metal M in the M salt and the reduced graphene oxide to be (3-8 mol): (40-80 mg);
(3) mixing the intermediate 2 with sodium hypophosphite, heating to react in an inert atmosphere, washing and drying after the reaction is finished to obtain the graphene/MPxA nanocomposite material.
4. The method as claimed in claim 3, wherein the concentration of graphene oxide in the reaction system is controlled to be 1-4mg/mL, the reaction temperature is 140-200 ℃, and the reaction time is 10-24h when the reduced graphene oxide hydrogel is prepared by hydrothermal reaction.
5. The preparation method according to claim 3, wherein the reaction temperature is controlled to be 120-180 ℃ and the reaction time is 3-12h when the reaction is heated after standing; when the reaction is heated in the inert atmosphere, the reaction temperature is controlled to be 300-400 ℃, the heat preservation time is 1-4h, and the heating rate is 5 ℃/min.
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